(1) Field of the Invention
The present invention relates to the field of electric machines. It relates to a separately excited reversible electric machine having at least one exciter unit formed by a primary magnetic circuit and two secondary magnetic circuits. Such an electric machine may be single-phase or polyphase.
(2) Description of Related Art
An electric machine that transforms electrical energy into mechanical energy, e.g. for the purpose of propelling a vehicle, is referred to as a “motor”. An electric machine transforming mechanical energy into electrical energy, e.g. for generating electricity, is referred to as a “generator”. Generators include both alternators that supply alternating current (AC) and dynamos that supply direct current (DC).
An electric motor may be adapted to be powered by DC or by single-phase or polyphase AC, for example three-phase AC. Likewise, an alternator may be adapted to generate AC that is single-phase or polyphase.
Nevertheless, it is preferable for polyphase AC to be balanced in order to enable operation to take place smoothly and without jolting the electric machine. Such balanced polyphase AC then forms a balanced electrical system with at least three phases and it is characterized in particular by the fact that the sum of the complex voltages (or currents) of each of the phases is zero, but without the amplitudes of the voltages (or currents) of each of the phases being zero simultaneously. Furthermore, the same phase shift is present between each of the phases of this alternating current.
An electric machine is said to be “reversible” when it can be used either as a motor or as a generator. Any electric machine can be reversible, with the distinction between the motor and generator functions being made only with reference to the purpose and utilization of the electric machine. The term “motor-generator” is also used if both functions are available on the electric machine.
The motors currently in use may be rotary, i.e. they produce angular movement and/or torque, or they may be linear, i.e. they produce linear movement and/or force.
In contrast, generators are essentially rotary.
A rotary electric machine is an electromechanical device having at least one stator that is stationary and at least one rotor that rotates relative to the stator and that may be located inside and/or outside the stator. The rotor is caused to rotate by interaction between two magnetic fields, associated respectively with the stator and with the rotor, thereby creating magnetic torque on the rotor. These are referred to respectively as a “stator magnetic field” and as a “rotor magnetic field”.
Below, this description is limited to rotary electric machines, so the term “electric machine” is used more simply to designate a rotary electric machine. Likewise, the term “electric motor” designates a rotary electric motor, and the term “generator” designates a rotary electricity generator.
The various electric machine technologies differ essentially in the way in which these stator and rotor magnetic fields are generated.
For example, in a DC electric motor, the stator has magnetic elements, that may be permanent magnets or non-permanent magnets, which are referred more commonly as electromagnets and which are constituted by one or more coils of electrical conductors powered by DC. The term “winding” is used below to designate a set of one or more electrical conductor coils. Whether permanent or non-permanent, each magnet has two poles, a north pole and a south pole, and as result a stationary stator magnetic field is created. In contrast, the rotor has non-permanent magnets that are constituted by a winding that creates a rotor magnetic field when it is passing DC. During rotation of the rotor, a rotary commutator serves, at least once per revolution, to reverse the direction of the DC passing through the rotor winding, thus reversing the poles of the non-permanent magnets of the rotor and thereby changing the direction of the rotor magnetic field.
Thus, a shift between the stator and rotor magnetic fields causes a magnetic torque to be applied to the rotor, for example, with a north pole of the stator repelling a north pole of the rotor, while attracting a south pole of the rotor. This causes the rotor to turn relative to the stator.
A main drawback of such a DC electric motor lies in the electrical contacts needed between the rotor winding and the rotary commutator. These contacts, which may be obtained by means of brushes, for example, can create electric arcs that lead in particular to wear and to interference that consequently require the electric machine to be maintained frequently. Furthermore, that type of electric motor is not suitable for high speed rotation and it consumes energy as a result of friction, thereby reducing its efficiency. Finally, it can be complex to make.
Those drawbacks are eliminated by the technology of so-called “brushless” motors.
The rotor of such an electric machine has one or more permanent magnets, while the stator has a winding constituting non-permanent magnets. Such a machine may also have means for determining the position of the rotor, e.g. by using a sensor, together with an electronic control system for switching electric current. An alternating electric current is thus passed through the stator winding. As a result, the electronic control system can determine the orientation and the direction of the stator magnetic field relative to the rotor magnetic field, and can consequently cause the rotor to rotate relative to the stator, with the rotating stator field entraining the rotor field.
Furthermore, within the stator winding, one or more coils may be grouped together in order to form different phases of the stator, each phase having the same shift relative to the other phases. In motor mode, each phase is powered by one of the phases of polyphase AC and generates a respective stator magnetic field, each stator magnetic field associated with any one phase being likewise shifted relative to the other stator magnetic fields associated with the other phases. When the stator magnetic fields are derived from the same polyphase AC in a balanced electrical system, they act together to form a single stator magnetic field referred to as the stator resultant field. By interacting with the rotor field, this stator resultant field can generate torque, and consequently can cause the rotor to rotate relative to the stator.
Likewise, in generator mode, rotation of the rotor causes the rotor field to rotate and causes a rotating stator resultant field to appear, which can be resolved into one magnetic field for each phase of the stator, thereby causing polyphase AC to appear.
AC electric machines include both synchronous electric machines and asynchronous electric machines.
Synchronous electric machines, which include brushless motors, have a rotor made up of one or more permanent magnets and a stator having a winding with a plurality of coils capable of forming one or more phases. When the coils of the stator winding are passing one or more alternating currents of a balanced polyphase electrical system, they create one or more stator magnetic fields that may be rotating, producing a stator resultant field that interacts with the rotor magnetic field at the synchronous frequency of the machine, thereby generating torque for rotating the rotor relative to the stator.
Conversely, when the rotor is rotated by external mechanical power, that causes the rotor magnetic field to rotate, thereby causing one or more alternating electric currents to appear and flow in the stator winding, and consequently creating one or more magnetic fields.
The permanent magnets of the rotor may be replaced by a winding that is powered by DC, thereby constituting non-permanent magnets, and thus creating a rotor magnetic field. The DC may be delivered by a current source such as a battery or a capacitor.
The frequency of rotation of the rotor of a synchronous electric machine is proportional to the frequency of the AC applied to the stator. Likewise, the frequency of the AC generated in a synchronous generator is proportional to the frequency of rotation of the rotor. A synchronous machine is often used as a generator, e.g. as an alternator in power stations.
Asynchronous electric machines have a rotor with a winding having coils that may be short circuited, for example, and a stator with a winding constituting non-permanent magnets. When the stator winding is passing AC, it creates one or more stator magnetic fields that may be rotating and that provide a stator resultant field that causes electric current to appear in the rotor winding, thereby generating a magnetic torque on the rotor and consequently causing the rotor to rotate relative to the stator.
Conversely, rotation of the rotor generated by external mechanical power causes AC to appear and flow in the winding of the stator. For this purpose, it is necessary to connect the electric machine to a power supply, e.g. including at least one converter and a battery, in order to supply it with the reactive energy it needs in order to operate in generator mode.
Although the frequency of rotation of the stator magnetic field is proportional to the frequency of the AC passing through the stator winding, the frequency of rotation of the rotor of an asynchronous electric machine is generally not proportional to this frequency of the AC, and a slip speed appears between the rotor and the stator magnetic field. Likewise, the frequency of the AC generated in an asynchronous generator is not necessarily proportional to the frequency of rotation of the rotor.
For a long time, asynchronous machines were used only as electric motors, e.g. in transport for the purpose of propelling ships and trains, and also in industry for machine tools. But nowadays, by using power electronics, such electric machines are also used as generators, e.g. in wind turbines.
Furthermore, the use of such reversible electric machines on board vehicles, such as cars or rotary wing aircraft, is becoming more widespread in order to provide a hybrid power plant that makes use of two types of energy for propelling the vehicle, both thermal energy and electrical energy. Nevertheless, such use is presently limited by certain constraints, such as the power-to-weight ratio of such electric machines and of electrical energy storage means.
Whatever the type of a reversible electric machine, a magnetic flux circulates between the rotor and the stator through the various permanent or non-permanent magnets of the rotor and of the stator, the flux being directed by the magnetic poles of the magnets. Specifically, such a magnetic flux generally circulates from a north pole towards a south pole through an airgap situated between each pole of the stator and the rotor, and also between a south pole and a north pole of the stator and of the rotor.
Furthermore, the magnets of the rotor, whether permanent or non-permanent, can be directed in two different ways leading to at least three types of electric machine.
Firstly, the magnets may be directed perpendicularly to the axis of rotation of the electric machine, i.e. the two poles of each magnet lie on a direction that is perpendicular to the axis of rotation. The magnets are then said to be radially oriented or more simply the magnets are said to be radial. An axial magnetic flux is then created in the airgap of the electric machine, i.e. parallel to its axis of rotation. The machine is then said to be an “axial” electric machine.
Secondly, the magnets may be directed parallel to the axis of rotation of the electric machine, i.e. the two poles of each magnet are directed parallel to the axis of rotation. It is then said that these magnets are axially oriented, or more simply that these magnets are axial. A radial magnetic flux is then created in the airgap, i.e. perpendicularly to the axis of rotation. Such a machine is said to be a “radial” electric machine.
These various orientations of the magnets serve to direct the magnetic flux circulating in the electric machine, which may then be axial, in a first type of electric machine, e.g. electric machines having a disk rotor, or radial in a second type of electric machine, e.g. electric machines having a cylindrical rotor. In a third type of electric machine, it is also possible to use both radial and axial magnets within the same electric machine such that magnetic flux is created both axially and radially. Such magnetic flux is then said to be “multiple-airgap” magnetic flux.
Furthermore, the term “homopolar machine” is also used to designate an electric machine in which the magnetic flux passes at least locally in an axial direction.
Nowadays, electric machines use a variety of configurations and orientations for magnetic flux in order to satisfy customer needs better, both in terms of performance and in terms of dimensions. For example, permanent magnetic machines with axial flux and high torque are axially shorter and radially larger, whereas radial magnetic flux machines are smaller radially and longer axially.
Furthermore, the power-to-weight ratio of such electric machines, i.e. their power divided by their weight, and also their cost of fabrication, varies depending on the magnetic flux configurations used.
Homopolar and transverse flux electric machines with permanent magnets are nowadays preferred because of high magnetic torque, due in particular to using permanent magnets, and due to a high performance-to-cost ratio compared with other machine technologies.
Nevertheless, the use of permanent magnets involves several drawbacks, in particular demagnetization of the permanent magnets at high temperature, thereby narrowing the range of applications in which such electric machines can be used. Furthermore, permanent magnets give rise to resisting torque in the generator function, thereby reducing the efficiency of the electric machine. In addition, the use of permanent magnets requires a generator electric machine to be regulated using high power electronics in order to take account of the impossibility of de-exciting the permanent magnet of such an electric machine, and such electronics can lead to problems of safety. However, a machine with winding excitation is regulated very simply and does not require high power electronics.
Furthermore, a short circuit on one of the stator phases of such an electric machine with permanent magnets can lead to a high level of resisting torque that can lead to consequences that are severe, and possibly to operation of the machine being blocked. Other major risks exist, such as breakdown of the power electronics when subjected to excess voltage, or such as a fire starting in the event of a short circuit.
Finally, permanent magnets, when they are fabricated using rare earths, are expensive, and may even become unavailable in the not so distant future.
Homopolar and transverse flux electric machines without permanent magnets, such as those described in Document FR 2 969 409, avoid some of those constraints associated with permanent magnets, but they do not make it possible to achieve equivalent performance. The rotor of such an electric machine includes strips of ferromagnetic material that are magnetized by a rotor exciter winding, also referred to as a “primary magnetic circuit” that is positioned on the stator of the electric machine. The primary magnetic circuit is powered by DC and thus generates constant rotor magnetic flux. Such an electric machine is referred to as a “separately excited electric machine”. The stator of the separately excited electric machine also has two stator exciter windings, referred to as “secondary magnetic circuits”, and two yokes having teeth surrounding each secondary magnetic circuit. The secondary magnetic circuits are powered with AC and thus generate stator magnetic flux, with electricity flowing in opposite directions in the two secondary magnetic circuits. Furthermore, these secondary magnetic circuits lie on either side of the primary magnetic circuit, i.e. one secondary magnetic circuit is positioned on each side of the primary magnetic circuit. As a result, the rotor is caused to rotate by interaction between the rotor and stator fluxes, each strip being simultaneously magnetically attracted by a tooth of a yoke of one secondary magnetic circuit and magnetically repelled by another tooth of a yoke of the other secondary magnetic circuit, the electric currents that flow in the two secondary magnetic circuits flowing in opposite directions.
Nevertheless, that machine requires two secondary magnetic circuits in order to operate and a number of poles on the strips of the rotor that is equal to the number of pairs of teeth on each secondary magnetic circuit. As a result, the number of strips of the rotor is half the number of teeth of each secondary magnetic circuit, so the torque supplied by that electric machine is half the torque supplied by an electric machine of identical size having permanent magnets on the rotor. Furthermore, the weight of such an electric machine is increased by the use of three windings, namely two secondary windings and one primary winding, the winding of the primary magnetic circuit generally being heavier than the windings of the secondary magnetic circuits. Thus, the drawbacks associated with using permanent magnets can be eliminated using such a separately excited electric machine, but to the detriment of the performance of that electric machine.
Furthermore, the technological background of the invention includes the following documents: U.S. Pat. No. 2,417,880 and GB 917 263.